1. Field of the Invention
The present invention pertains to reactive organopolysiloxanes (silicones) bearing alkoxy groups and reactive organic groups which dominate the overall reactivity of the reactive silicone. The reactive silicones are prepared through hydrolytic condensation of hydrolyzable precursors. The invention further pertains to curable compositions containing the reactive organopolysiloxanes, and to their use, particularly in coatings and encapsulants.
2. Description of the Related Art
Silicones having reactive organic functional groups such as hydroxyalkyl, aminoalkyl, isocyanatoalkyl and the like are known. Such reactive silicones may be prepared, for example, by hydrosilylating an ethylenically unsaturated compound also bearing a desired reactive group, for example allylamine or isocyanatoethylmethacrylate with a silane or polysiloxane bearing silicon-bonded hydrogen (≡Si—H). A desirable characteristic of these reactive silicones is that they react exclusively through the reactive organic functionality, or in other words, have “defined reactivity.” However, a disadvantage is that more expensive Si—H functional organosilicon compounds must be used to prepare them, and that ethylenically unsaturated compounds bearing the desired reactive group may not be available, may not have the desired stability, or are available at only relatively high cost.
A further disadvantage is that hydrosilylation generally employs a noble metal hydrosilylation catalyst, generally a platinum-based catalyst, which adds to the expense. If the hydrosilylation reaction is not complete, unreacted ethylenically unsaturated reactants must be removed, e.g. by subjecting the product mixture to stripping or vacuum, which is not always effective unless the temperature is raised. For some reactive groups, however, raising the temperature is contraindicated, as the reactive functional groups may react or condense. Furthermore, if the final product contains unreacted Si—H groups, these may give rise to storage problems, especially if water is present. Reaction with water can liberate explosive hydrogen gas.
In U.S. Pat. No. 5,814,703, highly branched silicones having aminoalkyl, epoxyalkyl, or ethylenically unsaturated groups are prepared, not by hydrosilylation, but by hydrolytic condensation of a functional dialkoxysilane or trialkoxysilane with a non-functional dialkoxysilane or trialkoxysilane, optionally together with tetra-alkoxysilanes. These highly branched reactive silicones contain minimally 10 mol percent of “T-units,” RSiO3/2, which form branching sites. Moreover, they contain a limited amount of non-functional hydrocarbon groups relative to the number of silicon atoms. Due to these requirements, and as a result of the preparation method, in addition to the desired reactive functional group, the products contain a large quantity of non-hydrolyzed alkoxy groups. These reactive silicones may be used to form hard coatings when admixed with a non-functional polymer resin, or preferably, a reactive, crosslinkable polymer resin.
It has been found, however, that reactive silicones such as those disclosed in U.S. Pat. No. 5,814,703, have numerous drawbacks. First, the relatively high proportion of alkoxy groups allows the silicone, once the organic reactive groups have reacted, to further crosslink in the presence of moisture, which is unavoidable in coatings and articles intended for normal use. Thus, the chemical bonds formed are only partially the result of reaction of the intended organic functional groups. “Designed reactivity” under such conditions is impossible to achieve. Moreover, under conditions of high humidity, the alkoxy groups may react even prior to reaction of the organic functional groups, decreasing mobility of the growing polymer chains to the extent that a proportion of the functional groups may remain unreacted. Furthermore, the products, particularly when used in sections thicker than thin films, show evidence of cracking, shrinkage, and voids (from outgassing of condensation reaction alcohol) which may occur even as early as during initial cure. Such compositions are completely unsuitable as encapsulants for electronic devices, for example. Finally, these reactive silicones display poor compatibility with many polymers, as a result of which a homogeneous coating composition is difficult or even impossible to obtain, or which may be subject to phase-out into silicone-rich and silicone-poor regions in the cured product.
It would be desirable to provide reactive silicone polymers by a method which avoids hydrosilylation and its disadvantages, yet provides a greater degree of defined reactivity. It would be further desirable to provide reactive silicone resins which are flexible and exhibit little tendency to crack or develop voids during cure or thereafter, and which exhibit greater compatibility with organic polymers.